Electrosensitive recording apparatus



April 1968 c. s. REIS 3,377,599

ELECTROSENS ITI VE RECORDING APPARATUS Filed Oct. 22, 1964 WRITING 21CONTROL WRITING r23 CONTROL HEAT SOURCE INVENTOR CHARLES S. REIS BY acswag ATTORNEY United States Patent 3,377,599 ELECTROSENSITIVE RECORDINGAPPARATUS Charles S. Reis, Mountain View, Calif., assignor toHewlett-Packard Company, Palo Alto, Calif., a

corporation of California Filed Oct. 22, 1964, Ser. No. 405,800 6Claims. (Cl. 346-74) ABSTRACT OF THE DISCLOSURE of energy in the form ofan applied electrical writing,

signal.

It is an object of the present invention to provide an improvedelectrosensitive recording medium which responds to lower voltagewriting signals of the order of 1 to 3 volts.

It is another object of the present invention to provide anelectrosensitive recording medium which may be biased below recordinglevels by a source of energy to decrease the power required in thewriting signal to produce a recording.

It is another object of the present invention to provide an improvedmethod of electrosensitive recording.

In accordance with the illustrated embodiment of the present invention,a very low voltage electrosensitive recording medium is produced bytreating a conductive base layer disposed on backing material to form anohmic contact between a surface layer of heat-sensitive material and thebase layer. This overcomes the effects of any barrier layers which formbetween the base layer and the surface layer and presents sufiicientlyhigh conductivity to applied writing signals to produce contrastingmarks in response to applied wriling signals as low as 1 to 3 volts.

These and other objects of the invention will be apparent from a readingof this specification and an inspection of the accompanying drawingwhich shows electrosensitive medium and recording apparatus according tothe present invention.

In the drawing, relative motion of the recording medium 9 with respectto the electrodes 11, 13 may be produced by moving the recording medium9 and electrodes 11, 13 at dissimilar velocities. Writing signals fromsource 15 are applied to the electrodes 11, 13 disposed in contact withthe surface of recording medium 9 through switches 17, 19 which areactuated by writing controls 21, 23. Contrasting lines or marks 25 areproduced on the medium 9 in response to the writing signals as theelectrodes pass along the surface. The electrosensitive recording medium9 is prepared according to one embodiment of the present invention asfollows: aluminum is alloyed with indium in an amount typically of about1 to percent but less than 20 percent by weight. The alloy isvapor-deposited or laminated onto suitable backing material 27 such aspaper or plastic film using well-known methods to form a thin conductivebase layer 29. Other elements such as gallium, tin or other tri-valentelectron donors may also be used in place of (or in addition to) theindium.

A conductive surface coating 31 comprising a heat: sensitive materialwhich changes color at elevated temperatures is deposited on the baselayer 29 using such conventional techniques as spraying or rolling. Thesurface coating 31 may be prepared according to one embodiment of thepresent invention by combining grams of indium-doped zinc oxide whichacts as a white pigment as well as an electrical conductor, 62.5 gramsof a suitable binder such as Lucite No. 2040, 137 milliliters of asolvent such as xylene or toluene and 25 grams of heatsensitivesubstance formulated using four parts by weight of a heavy metal saltsuch as nickel acetate or nickel stearate to one part by weight of analkaline earth metal sulfide such as calcium sulfide, barium sulfide orthe like.

Other heat-sensitive substances may be formulated using 7.5 parts byweight of nickel palmitate, 2.5 parts by weight of barium or calciumsulfate and 1 part by weight of stearic or palmitic acid. Still anotherheatsensitive substance may be formulated using 5 parts by weight of astearate of a metal such as cobalt, copper, lead, magnesium, manganese,mercury, nickel or zinc and 1 part by weight of diphenylcarbozone. Ofcourse, it will be appreciated by those skilled in the art that anyheat-sensitive material including azo, diazo and leuco dyes (the latterdye being less light-sensitive than the azo and diazo dyes) may be usedin the surface coating material.

This coating material is rolled onto the base layer 29 to a wetthickness of about 3 mils and is dried at a temperature below thethreshold value at which the color transiaion in the heat-sensitivematerial takes place. The surface layer 31 thus formed comprises amatrix of heatsensitive and electrically conductive materials which arebonded to the base layer 29.

It is believed that the base layer 29 which is an alloy of aluminumbecomes coated with a discontinuous film of oxide when exposed to airduring the fabrication process. This oxide layer acts as an insulatorwhich decreases the conductivity of the recording medium from thesurface layer 31 to the base layer 29. This increases the amplitude ofwriting signal necessary to provide sufficiently high current density toproduce a contrasting record on the surface layer. The indium in thealloy base layer is believed to retard development of an oxide coatingand to wet the contiguous surfaces of the base layer and surface layerthrough regions of discontinuity in the oxide film, thereby forming alow ohmic resistive contact between the two layers. A minute amount ofdiffusion of the indium into the surface layer is also believed tooccur, thereby further increasing the conductivity of the ohmic contact.The high conductivity of the recording medium thus reduces the-writingsignal amplitude required to produce a contrasting record on the surfacelayer 31.

' The writing signal amplitude required to produce a contrasting recordmay be reduced further according to one embodiment of the presentinvention by providing a biasing signal for the heat-sensitive surfacelayer. As is commonly known, leuco dyes and other mentionedheatsensitive compounds used in the surface layer 31 change composition,and hence, change color rather abruptly with temperature. Thus thesurface layer 31 may be elevated in temperature by heating element 30disposed near the electrodes 11, 13 to a temperature just below thethreshold value at which the color transition in the dye occurs. Thistransition may occur typically within a 5 C. range so that the internalor joule heating provided by the writing signal need only produce asmall increment in temperature. Considerably less power per electrode isthen required from the writing signal to elevate the temperature ofrecord regions on the surface layer 31 above the transition temperatureto produce a contrasting record. Thus electrosensitive recording may beachieved using the recording medium and apparatus of the presentinventionwith writing signals levels, typically 100 may be due in partof a few volts at very low power milliwatts or less per electrode. Thisto the highly conductive ohmic contact formed between the base andsurface layers by the indium in the base layer alloy and in part to thelocally applied heat which biases the heat-sensitive material in thesurface layer to a temperature just below the temperature at which theheat-sensitive material undergoes a color transition.

I claim:

1. Recording apparatus comprising:

a base layer of conductive material including an alloy of aluminum andan element selected from the group comprising indium, gallium and tin;

a conductive surface layer disposed on said base layer and including aheat-sensitive material which changes color at elevated temperatures;

said element diffusing into said surface layer to form an ohmic contactbetween said surface layer and said base layer;

whereby a writing signal applied to said surface coating providescurrent flow therethrough with suflicient current density to elevate thetemperature of local regions thereof to produce a color change in suchregions.

2. Recording apparatus as in claim 1 wherein said surface layer includesa heat-sensitive material selected from the group comprising azo, diazoand leuco dyes and a material which shows high electrical conductivity.

3. Recording apparatus comprising:

a backing layer;

a conductive layer on said backing layer including an alloy of aluminumand iridium in a ratio of less than one part indium to four partsaluminum;

a conductive surface layer disposed on said conductive layer;

said surface layer including a heat-sensitive material selected from thegroup comprising azo, diazo and leuco dyes which change color atelevated temperatures;

the indium in said conductive layer diffusing into said surface layer toform an ohmic contact between said surface layer and said conductivelayer;

whereby a writing signal applied to said surface layer provides currentflow therethrough with sufiicient current density to elevate thetemperature of local regions thereof to produce a color change in suchregions. I

4. Recording apparatus comprising:

a base layer of conductive material;

a conductive surface coating disposed on said base layer and including aheat-sensitive material which changes color at elevated temperatures;

a heat source disposed near said surface coating in a selected regionfor heating said surface coating to a temperature below the elevatedtemperature at which said heat-sensitive material changes color;

a source of writing signal; and

an electrode connected to said source and disposed to apply a writingsignal to said surface coating with sufficient amplitude to produce acurrent flow therethrough with sufiicient current density to increasethe temperature of local regions of the surface coating near theelectrode to a temperature above the elevated temperature at which saidheat-sensitive material changes color, thereby to produce a contrastingrecord in such regions.

5. Recording apparatus as in claim 4 wherein:

said heat-sensitive material in said surface layer is selected from thegroup comprising azo, diazo and leuco dyes.

6. Recording apparatus comprising:

a base layer including an alloy of aluminum and an element selected fromthe group comprising indium, gallium and tin;

a conductive surface coating disposed on said base layer and including aheat-sensitive material which changes color at elevated temperatures;

said element being diffused into the surface coating to form an ohmiccontact between said surface layer and said base layer;

a heat source disposed near said surface coating in a selected regionfor heating said surface coating to a temperature below the elevatedtemperature at which said heat-sensitive material changes color;

a source of writing signal; and

an electrode connected to said source and disposed to apply a writingsignal to said surface coating with sufiicient amplitude to produce acurrent flow therethrough with sufficient current density to increasethe temperature of local regions of the surface coating near theelectrode to a temperature above the elevated temperature at which saidheat-sensitive material changes color, thereby to produce a contrastingrecord in such region.

References Cited UNITED STATES PATENTS 1,844,199 2/1932 Bicknell 346 X2,680,062 6/1954 Sus 117-36.8 2,855,266 10/1958 James 346-76 X 3,089,9525/1963 Wartman 11736.8 X 3,152,321 10/1964 Peltzer 34674 3,158,50611/1964 Ellison 34676 X 3,167,444 1/1965 Baumann 11736.8 3,176,2783/1965 Mayer 34674 3,299,433 1/1967 Reis 34672 BERNARD KONICK, PrimaryExaminer.

T ERRELL W. FEARS, Examiner.

L. SCHROEDER, Assistant Examiner.

